Amtec Guide to Aluminium & Aluminium Alloy Corrosion

Introduction

Amtec consultants provide corrosion expertise in the field of metallic corrosion. This Guide is one of a series on our website dealing with a number of Corrosion & Coating Breakdown topics. Other pages are focused on areas such as marine corrosion, Industrial corrosion, Corrosion in Hot Water & heating Systems. Other guides deal with Copper Corrosion & Stainless Steel Corrosion.

Aluminium Corrosion

The corrosion behavior of Aluminium & it's alloys is controlled by the fact that like stainless steel it is a passive metal with a protective air formed film forming naturally on the surface. This naturally forming film is made up of two layers which normally have a total thickness of about 7 nanometers. The lowest layer is termed the barrier layer & it is amorphous and has no structure. The layer that grows on top of this is hydrated & less compact. It tends to grow at high humidities or with contact with water.

The air formed passive layers are very stable & result in very low corrosion rates in neutral environments. However the film dissolves in both acid & alkaline conditions. High corrosion rates can be expected below pH3 & above pH 9.

Common Aluminium Alloys

Aluminium alloys are generally separated into 8 distinct series which are designated 1xxx through to 8xxx. The classification is as set out below:

• 1000 Series Aluminium alloys are variations on pure aluminium. They are unalloyed & are generally more than 99% pure & have a combination of good electrical conductivity & chemical resistance.
• 2000 Series Aluminium alloys contain copper as the main alloying ingredient. They have moderately high strength but tend to be brittle & exhibit poorer corrosion resistance than other alloy series.
• 3000 Series Aluminium alloys containing silicon in combination with copper or magnesium. They con be age hardened to improve their mechanical properties. They have fairly good corrosion resistance but are particularly resistant to pitting corrosion.
• 4000 Series Aluminium alloys also contain silicon as the main alloying ingredient but do not contain copper or magnesium. Their main use stems from their lower melting point which confers good casting qualities.
• 5000 Series Aluminium alloys are base around magnesium. They have good resistance to sea water & find use in marine applications as they have both moderate strength & ductility. they are easy to weld & anodize well.
• 6000 Series Aluminium alloys contain both magnesium & silicon. they have good resistance to atmospheric corrosion and form an attractive film after anodising. They are relatively easy to roll extrude and forge.
• 7000 Series Aluminium alloys contain zinc as the majority alloying metal but may also contain additions of copper, magnesium chromium or zirconium. Some of the copper containing variants have a strong tenancy to exfoliation corrosion in the heat effected zone of welds. However allots such as 7075 are have good resistance to aggressive atmospheres.
• The 8000 series Aluminium alloys may contain tin, iron or silicon. They are relatively uncommon but can be rolled to thin sheets which gives them some use as fins for heat exchangers & as dishes or thin foils.

Corrosion Of Aluminium Alloys

Uniform Corrosion & Passive Films

The uniform corrosion of aluminium of aluminium only tends to occur in environments that are not of neutral pH. The corrosion develops as pits of about one micron in diameter which as spread fairly uniformly across the surface. In mildly aggressive environments corrosion rates in the order of microns per year are to be expected however at more extreme pH values corrosion rates may be up to 10,000 times higher.

Crevice Corrosion Of Aluminium

Crevice corrosion of aluminium & it's alloys is relatively rare as compared to stainless steel it exhibits a relatively low susceptibility as the crevices tend to become filled with aluminium corrosion products.

Pitting Corrosion Of Aluminium

Pitting corrosion of aluminium generally occurs at pH values close to neutral. The pits are usually rather obvious as they ten to contain large amounts of hydrated white alumina gel of the type shown in the photograph above. In general the pits tend to start within the first few weeks of exposure. Pitting develops as a two stage process with an initiation stage and a propagation stage. During the initiation stage the passive film is penetrated by aggressive ions such as chlorides. The number of initiation sites will exceed mainly thousand per square centimeter. These pits compete amongst themselves and only a very small number eventually propagating. Area where heat treatment or alloying variations are present can become exclusively cathodic and drive pit propagation even more strongly in nearby areas as shown in the photograph below:

Generally the rate of pit propagation decreases with time however sometimes a single pit will continue to grow which results in perforation.

In the photograph above it can be seen that the vertical areas have substantial more pitting than the horizontal areas.

Galvanic Corrosion Involving Aluminium

Aluminium is low down in the galvanic series close to zinc and will corrode if in electrical contact with most other engineering metals such as steel, stainless steel & copper alloys. In these cells aluminium forms the anode and the other metal the cathode. Relative surface area is a major issue so aluminium pop rivets will dissolve out very rapidly if used on copper components. The rate of galvanic corrosion depends both on the size of the driving voltage and the conductivity of the environment. Consequently it is more of an issue in marine & industrial environments than rural ones.

Deposition Corrosion on Aluminium

The photograph above shows an example of deposition corrosion causing worsening of pitting on a marine alloy. Copper ions have gone into solution within the pipe and have plated out on the aluminium surface. This has resulted in a much improved cathode which has accelerated the corrosion.

End Grain Attack On aluminium

This phenomenon is similar to exfoliation corrosion. The photograph at the very start of this guide shows an example. The photograph below shows exfoliation corrosion.

In both types of attack the corrosion travels along a number of planes running parallel to the rolling direction and leaves areas or sheets of relatively unaffected material in between. The corrosion products tend to push the sheets of sound material apart.

Filiform Corrosion

Filiform corrosion is form of surface breakdown normally associated with thin or transparent coatings on aluminium. It starts at edges or coating defect and travels beneath the coating in irregular tunnels of the type shown in the photograph above. It is a cosmetic problem & is rarely associated with substantial metal loss.

Surface Treatments & Surface Coatings For Aluminium

The most common surface modification to aluminium surfaces is anodising. There are three common types of anodising treatment and they are commonly known as types I, II, & III. Type I anodising is carried out in Chromic acid & is relatively uncommon due to environmental reasons but still persists in the aerospace industry. Types II & III anodising are carried out in sulphuric acid. Type II is the most common type for decorative finishes & as a pre-treatment prior to the application of organic coatings. The gold coloured component shown above has been sealed using a dye in the hot water sealing solution. The anodised keyboard shown below has been sealed without a dye in the sealing water.

Type III anodising is sometimes referred to as hard anodising as it produces a thick & tough abrasion resistant film. It is generally more expensive as the temperature of the anodising bath has to be carefully controlled.

Other corrosion & coatings guides can be found in our reference library.

For more in depth information contact Amtec: enquiries@amteccorrosion.co.uk